1. Field of the Invention
The present invention generally relates to semiconductor integrated circuits. More specifically, the present invention relates to methods and structures for facilitating proximity communication between semiconductor chips.
2. Related Art
Conductive electrical interconnections and transceivers facilitate reliable communications between integrated circuit (IC) chips. Because of packaging and manufacturing advantages, conductive interconnections typically dominate the interconnect hierarchy within computer systems. However, decreasing semiconductor line-widths and increasing on-chip clock speeds are putting pressure on the ability of traditional resistive wires to achieve the off-chip bandwidths necessary to fully utilize on-chip computational resources.
A new technique referred to as “proximity communication” overcomes the limitations of resistive wires by using capacitive coupling to provide communications between chips that are oriented face-to-face. This capacitive coupling can provide signal densities two orders of magnitude denser than traditional off-chip communication using wire-bonding or traditional ball-bonding, while the circuits and coupling structures remain fully-compatible with standard CMOS foundries. To communicate off-chip through capacitive coupling, on-chip circuits drive a high-impedance, capacitive transmitter pad. Such communication avoids impedance conversion and thereby reduces the power normally dissipated by off-chip driver circuits. Moreover, simple driver circuits and small chip-to-chip distances can significantly reduce the total chip-to-chip communication latency.
While proximity communication provides off-chip signaling bandwidth that scales with chip feature size, it also introduces topological constraints. The active sides of chips typically need to face each other with full or partial overlap, so that corresponding transmitter and receiver pads on opposing chips align both laterally and vertically. Since the strength of the capacitively-coupled signal voltage on a receiver pad is inversely proportional to the distance between the receiver pad and a corresponding transmitter pad, maintaining a minimal vertical “z-separation” is important for successful communication. However, achieving and maintaining such alignment and proximity is difficult, especially when working with rigid, noncompliant chips that typically experience large temperature variations during operation.
Hence, what is needed are structures and methods that facilitate inter-chip alignment for proximity communication without the limitations of existing approaches.